Cleanup vroot test and fix false positive.

The previous test was too complicated. Use the kernel binary
sysctl to exercise the get_user call instead. Besides being
simpler, it has the advantage of not crashing the device
if the vulnerability is present.

Credit for the idea comes from https://twitter.com/grsecurity/status/401443359912239105

Bug: 15408470
Change-Id: Ib77436a31fa3c55e15b56f6e1a7a89d1d39b81ee